This invention relates generally to in-core, gamma ray heat sensing means for determining local power generation in a nuclear reactor.
According to prior copending application, now U.S. Pat. No. 4,298,430, owned in common with the present application by the same assignee, several gamma sensor assemblies for local power measurement are disclosed wherein an elongated monolithic body of gamma ray heated material mounts thermocouples for measuring temperature differentials produced internally in the elongated body at a plurality of axially spaced measurement zones. The temperature differential is a direct function of the average volumetric heat flow rate for the measurement zone which is in turn directly related to local power generation. The disclosure in U.S. Pat. No. 4,298,430 is hereby incorporated herein by reference. Other related, prior copending applications, now U.S. Patents, owned in common herewith are U.S. Pat. Nos. 4,313,792 and 4,356,061.
In U.S. Pat. No. 4,298,430, aforementioned, the claims are directed to gamma ray sensors within which axial heat flow patterns are established to produce the temperature differential measurements for each measurement zone in a pressurized water type of nuclear reactor where space for insertion of the sensor is restricted to 7.5 mm. for example, to limit the outer diameter of the elongated sensor body. Such temperature differentials are produced at each measurement zone by a thermally resistive region or gap in the body enclosed by an outer heat sink tube in thermally conductive relation to the constant outer diameter portion of the elongated sensor body. In such an arrangement, the resulting axial heat flow pattern is such as to render the thermocouple junction in the high thermal resistance region hot and the junction in spaced adjacency thereto, cold. The high thermal resistance gap has an axial length which is directly related to the differential temperature signal output of the sensor, disregarding gap heat losses and assuming a uniform heat sink temperature for the sensor. Further, the outer diameter of the sensor body has no direct affect on the level of the signal output. It was therefore, believed that only with a sensor of the type having the axial heat flow pattern could a suitable signal output level be obtained despite installation space limitations, by appropriate selection of the axial gap length. However, the axial heat flow type of sensor does have signal reliability problems because of axial gap heat losses resulting from deterioration of the high thermal resistance in the axial gap and deviations in heat sink temperature as a result of poor contact developed between the sensor body and the outer heat sink tube.
It is therefore, an important object of the present invention to provide a gamma sensor for measuring local power generation in a nuclear reactor, that is more reliable with respect to signal error and yet provide an adequate output signal level as well as to exhibit the necessary physical strength under installational conditions. An additional object is to provide such an improved gamma sensor which retains the attribute of direct electrical calibration.